This invention relates to the use of solid carbonaceous fuels such as coal as a fuel source in a process for making glass or similar fusion processes and to the use of mixtures of solid or liquid fuels with the raw materials.
It is well known that in regions where coal is available it is usually the cheapest source of energy relative to other traditional energy sources such as natural gas, fuel oil, and electricity. Therefore, it has been suggested that coal be used as a fuel source for melting glass and the like. Examples of such proposals may be seen in U.S. Pat. Nos. 3,969,068 and 4,006,003. However, the use of coal to fuel direct fired process furnaces has been found to have certain drawbacks that have discouraged its widespread use. A major drawback is the ash content of coal. When coal is combusted with an overhead burner in an open hearth type furnace conventionally employed to melt glass, substantial amounts of ash are entrained in the exhaust gas stream which can cause the regenerators to become plugged and which necessitates removal of the ash from the exhaust gas before it can be discharged to the atmosphere. Some of the ash becomes deposited on the walls of the melting chamber where it melts to a liquid slag that runs down the walls of the vessel into the melt. The runnage of molten slag has a deleterious affect on the refractories of the furnace, and the molten slag entering the melt introduces unwanted compositional variations and inhomogeneities into the product material. The slag often has a high iron content relative to glass, and runnage of the slag into the melt can cause undesirable streaks of coloration. These problems have discouraged the use of coal as a direct fuel for melting products for which uniformity of composition is an important consideration. This is particularly the case with flat glass, where compositional variations cause optical distortion in the product glass.
A drawback to the use of coal or other carbonaceous fuel in admixture with the raw materials, particularly when melting clear glass, is that carbon in amounts sufficient to provide significant energy to the melting process also has a reducing effect on the melt, and iron and sulfur present in a reduced glass cause brown coloration. Moreover, coal itself contributes iron and sulfur to the melt. Small amounts of powdered coal (typically less than 0.1% by weight) have been included in clear glass batch to aid the melting process, but such amounts are not significant energy sources, and larger amounts were considered detrimental. Even when brown glass is being produced, the amount of carbon employed would not be considered a significant fuel contribution.
U.S. Pat. No. 3,294,505 discloses melting glass in a bed of batch briquettes and coke. The process is restricted to a relatively narrow group of low viscosity glass compositions for low quality applications. Additionally, it would be desirable to avoid the cost of agglomerating the batch.
In commonly assigned, copending U.S. application Ser. No. 624,879 filed June 27, 1984, now U.S. Pat. No. 4,551,161, there is disclosed a technique of wetting glass batch with fuel oil. Only a minor portion of the energy requirement of the melting process is supplied by the fuel oil.
Another problem with using coal and some other carbonaceous fuels is that such fuels contain relatively volatile hydrocarbon fractions that are driven off and escape with the exhaust gas if heated before ignition. This is a problem particularly if it is desired to preheat raw materials in admixture with carbonaceous fuel. Also, feeding carbonaceous fuels in a non-atomized form to a combustion zone generally produces smoke-laden exhaust that is environmentally undesirable. After-burning or otherwise treating the exhaust gas or carbonizing the fuel in a preliminary operation are costly options that are preferably avoided.